Authorities in California say an elderly disabled woman has been killed in one of two massive wildfires in the northern part of the state that have overwhelmed firefighters with their speed and intensity.

The body of 72-year-old Barbara McWilliams, who suffered from advanced multiple sclerosis, was discovered in the rubble of her home near California's world-famous Napa Valley winemaking region. Firefighters dispatched to rescue McWilliams could not reach her house because the neighborhood was engulfed in flames.

The fire which has been dubbed the Valley fire, has destroyed 400 homes and and over 24,000 hectares of land since erupting last Saturday, and is threatening another 9,000 structures. The inferno was only about 10 percent contained as of late Monday night.

About 300 kilometers to the east, another fire was raging in the Sierra Mountains, having destroyed 135 homes by midday Monday.

The two fires are among 12 large wildfires burning in northern California. Emergency officials say dry vegetation caused by a four-year long drought plus a record heat wave is the major cause of the fires. Governor Jerry Brown, who has issued a state of emergency in order to free up resources to fight the blazes, told reporters Monday his state is in a battle with "Mother Nature."

"These fires are acting more aggressively, more unpredictably," Brown said. "It's scary stuff," he said and praised firefighters for showing a lot of courage.

The governor said the flames are spreading faster than what the computer models predicted. He said climate change will only lead to more fires in the future.

Forecasters are predicting cooler weather and higher humidity for the northern region for the rest of the week which will help the firefighters.